1. Field of the Invention
The present invention relates generally to optical fibers and particularly to coatings applied to the fibers.
2. Technical Background
Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has had a significant impact in the local area networks (i.e., for fiber-to-home uses), which has seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver ever greater volumes of information in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in home and commercial business for internal data, voice, and video communications has begun and is expected to increase.
The fibers used in local networks are directly exposed to harsh conditions, including severe temperature and humidity extremes. Prior coatings for optical fibers did not perform well under such adverse conditions, the need existed for the development of higher performance coatings to address the wide and varied temperature and humidity conditions in which fibers are employed. Specifically, these coatings possessed thermal, oxidative, and hydrolytic stability which is sufficient to protect the encapsulated fiber over a long life-span (i.e., about twenty-five or more years).
Optical fibers typically contain a glass core, a cladding, and at least two coatings, i.e., a primary (or inner) coating and a secondary (or outer) coating. The primary coating is applied directly to the cladding and, when cured, forms a soft, elastic, and compliant material which encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled. Stresses placed upon the optical fiber during handling may induce microbending of the fibers and cause attenuation of the light which is intended to pass through them, resulting in inefficient signal transmission. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the primary coating, and others for the secondary coating. The modulus of the primary coating must be sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber, which can induce microbending and consequent inefficient signal transmission. This cushioning effect must be maintained throughout the fiber""s lifetime.
Because of differential thermal expansion properties between the primary and secondary coatings, the primary coating must also have a glass transition temperature (Tg) which is lower than the foreseeable lowest use temperature. This enables the primary coating to remain elastic throughout the temperature range of use, facilitating differences in the coefficient of thermal expansion between the glass fiber and the secondary coating.
It is important for the primary coating to have a refractive index which is different (i.e., higher) than the refractive index of the cladding. This permits a refractive index differential between the cladding and the primary coating that allows errant light signals to be refracted away from the glass core.
The cost to produce coated optical fibers, with the above properties, is largely dependent on the draw tower line speed and draw utilization. A limiting factor in the operation of a draw tower line speed is the rate of cure of the primary and secondary coatings applied to the fibers. Under cured coatings can yield unwanted fiber defects, which may lead to functional problems with the resultant coated fiber. Previous methods to improve the rate of cure or rate of polymerization include the use of wholly acrylated coating systems, the use of highly efficient photoinitiating systems, and increases in UV radiation. Photoinitiated polymerization reactions generally follow the relationship:
Rp=kp[M]("PHgr"xcex5Io[A]b/kt)xc2xd
Rp: rate of polymerization; kp: propagation rate constant; [M]: concentration of reactive functional group; "PHgr": quantum yield for initiation; xcex5: molar absorptivity; Io: incident light intensity; [A]: concentration of photoinitiator; b: thickness of reaction system (coating thickness); and kt: termination rate constant.
Formulation efforts to maximize the rate of polymerization include the use of reactive monomers, oligomers, and mixtures thereof with high propagation rate constants, the use photoinitiators with high photoinitiating efficiencies, and selecting components that would not increase the tendency toward chain termination or chain transfer. Chain transfer agents may not decrease the rate of polymerization, but will reduce the degree of polymerization.
One aspect of the invention relates to an optical fiber which includes a glass fiber and a primary coating encapsulating and in contact with the glass fiber. This primary coating is the cured product of a polymerizable composition that includes a monomeric component with an oxyglycidyl (meth)acrylate.
Another aspect of the invention relates to a method of making an optical fiber in accordance with the present invention. This method involves providing a glass fiber, coating the glass fiber with a primary polymerizable composition that includes a monomeric component with an oxyglycidyl (meth)acrylate. The coating composition is polymerized under conditions effective to form a primary coating over the glass fiber.
The coating composition of the invention has the advantage of having an excellent polymerization rate. Coating an optical fiber with the coating of the invention has the advantage of increasing the draw speed and increasing the rate of production of the resultant optical fiber.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawing.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawing is included to provide a further understanding of the invention, and is incorporated in and constitutes a part of this specification. The drawing illustrates an embodiment of the invention, and together with the description serve to explain the principles and operation of the invention.